1. Field of the Invention
The present invention relates to an SRPP circuit (shunt regulated push pull).
2. Description of the Related Art
FIG. 1 shows a case in which a sixth order low-pass filter is formed of an active filter. Amplifiers A1 to A3 as used are generally formed of voltage followers. The voltage followers A1 to A3 (amplifiers A1 to A3) may be formed of (1) emitter followers, (2) SRPP circuits, (3) operational amplifiers, or the like.
Incidentally, the emitter follower of (1) may be structured, for example, as shown in FIG. 2. That is, the transistor Q1 has its emitter connected with a constant-current power transistor Q2 to form an emitter follower, its base supplied with an input signal voltage Vin and a base bias voltage VBB and its collector supplied with a supply voltage VCC. In this way, an output voltage or an output current is extracted from the emitter of transistor Q1.
In case of the emitter follower, assuming that the value of the collector current of the transistor Q2 is IC2, as indicated by a solid line in FIG. 3, the emitter current IE1 of the transistor Q1 varies in proportion with a signal voltage Vin with the value IC2 as a center. Thus, the amount of variation is extracted as an output.
Hence, as indicated by a broken line in FIG. 3, an output current as large as desired can be extracted within a range in which the characteristic of the transistor Q1 is permitted during a positive half cycle period. However, during a negative half cycle period, an output current larger than the IC2 cannot be extracted.
The SRPP circuit of (2) can be structured as shown in FIG. 4. In the SRPP circuit, a signal voltage opposite in phase to the input signal voltage Vin is outputted to the collector of the transistor Q1, and the signal voltage is supplied to the base of a transistor Q2 through a capacitor C1. Accordingly, since the transistors Q1 and Q2 are driven with phases opposite to each other, the emitter current of the transistor Q1 and the collector current of the transistor Q2 vary in directions opposite to each other, and the amount of its difference is extracted as the output current. Accordingly, the SRPP circuit can extract the large output current even during the negative half cycle period.
However, in case of the SRPP circuit, since the base input of the transistor Q2 is supplied from the collector of the transistor Q1 through the capacitor C1 for d.c. cutting, it is necessary to increase the value of the capacitor C1 in order to extract the large output even in a low frequency.
Accordingly, the SRPP circuit is not proper for implementation as an IC.
From the above viewpoint, the SRPP circuit shown in FIG. 5 has been proposed. That is, in the SRPP circuit, the collector output of the transistor Q1 is extracted through the transistor Q3 that constitutes the emitter follower, and a d.c. voltage is shifted by a constant voltage diode D1 before being supplied to the base of the transistor Q2.
Accordingly, in case of this SRPP circuit, since there is provided no element for limiting the frequency characteristic as in the capacitor C1, the frequency characteristic is excellent and a large output can be extracted even in the low frequency.
However, in case of this SRPP circuit, since the d.c. operating point of the circuit is set in accordance with the constant-voltage characteristic of the constant-voltage diode D1, if the supply voltage VCC varies, the operating current of the transistors Q1 to Q3 largely varies, thereby disenabling the satisfactory characteristic to be obtained. In particular, when the supply voltage VCC is low, such a tendency is high.
In addition, if the operational amplifier is used for the voltage follower, the number of operational amplifiers increases more as the degree is high, thereby making the circuit scale remarkably large. Also, if an amplifier for wide-band frequencies is used in correspondence with applied frequencies, current consumption is caused to increase.
The present invention has been made to solve the above problems with the conventional circuit.
To achieve the above object, according to the present invention, there is provided an SRPP circuit comprising:
a first transistor;
a resistor through which the collector of the first transistor is connected to a first reference potential point;
a second transistor having the same polarity as that of the first transistor, the collector of the second transistor being connected to the emitter of the first transistor, the emitter of the second transistor being connected to a second reference potential point; and
a third transistor having the polarity opposite to that of the first transistor, the emitter of the third transistor being connected to the collector of the first transistor and the collector of the third transistor being connected to the base of the second transistor, a bias voltage being supplied to the base of the third transistor;
wherein an input signal is supplied to the base of the first transistor, and an output signal is extracted from a node of the emitter of the first transistor and the collector of the second transistor.
Accordingly, since the third transistor operates with its base being grounded to supply the collector output of the first transistor to the base of the second transistor, the above circuit structure operates as an SRPP circuit.